Added get_non_essential_apps() method.

Addresses #50 

Added essential field to application base class. Wrote registry method to return non-essential apps.

Made files PEP8 compliant.

Tested calling this method, only notes application was returned as expected. 
I'm really confused as to what's going on in this method. What we wanted was a way to get the non-essential apps for each individual interview. In the current state, this method is just going to give us **all** non-essential apps, which isn't really adding much functionality.